This invention relates to a method of carrying out conversion between first signal sequence and second signal sequence in a differential pulse code modulation (DPCM) system and to an encoder and a decoder for use in carrying out the conversion. Mor particularly, this invention is applicable to an adaptive differential pulse code modulation (ADPCM) system. It is to be noted here that the conversion method may be an encoding method and/or a decoding method and that the encoder encodes the first signal sequence into the second signal sequence while the decoder decodes the second signal sequence into the first signal sequence.
As will later be described with reference to a few figures of the accompanying drawing, a conventional conversion method is described by Takao Nishitani et al in ICASSP 82 Proceedings, Volume 2 (May 1982), pages 960-963, under the title of "A 32 kb/s Toll Quality ADPCM Codec using a Single Chip Signal Processor." In the conventional conversion method, adaptive prediction is carried out in each of an encoder and a decoder by the use of an adaptive predictor so as to successively predict a current or predictive signal from preceding or past signals. The encoder encodes a first signal sequence with reference to the predictive signals into a second signal sequence which is produced in the form of predictive error signals between the first signal sequence and the predictive signals. On the other hand, the decoder reproduces the first signal sequence in response to the received second signal sequence with reference to the predictive signals derived from the received second signal sequence. Each predictive signal is adaptively varied in accordance with an input signal supplied to each predictor, so as to enable precise prediction.
As known in the art, the conversion method is very effective because electric power of the second electric signal sequence can be reduced as compared with that of the first signal sequence, when the first signal sequence carries a signal, such as an audio signal, having a strong correlation.
It should be noted here that operation of the encoder continues to be subject to the influence of the initial state for a long time after the beginning of operation. This is because the encoder has a feedback loop that includes the predictor. As a result, encoding precision or quality of the second signal succession is often degraded due to this dependence on the initial state. Moreover, the encoding quality may vary from one operation to another if the initial state of the encoder is different at the beginning of one operation from that at the beginning of the previous operation.
Such variation of the encoding quality adversely affects decoding operation of the decoder.